That's right, Valve has approved Seabrook's expansion to its canon to be sold for monetary value. In this case £7.50 / $9.99 / €9.99 (though pre-order are 10 per cent off).

Propsekt is set in the Nova Prospekt prison introduced in Half-Life 2. Series stalwart Gordon Freeman is cornered, so his Vortigaunt allies teleport in US Marine Adrian Shepherd - that's you! - to help out.

Seabrook has touted Prospekt as being comparable to Half-Life 2: Episode One in length, so somewhere between two and four hours depending on your playstyle. It will feature 13 levels, new music, voice acting, enemy AI, and a return to Xen, the alien planet introduced in Half-Life's final act.

Despite being set in the Half-Life universe, Prospekt won't require either parent game to operate as it's being sold as a standalone entity (and was made by a standalone entity, for that matter).

"I can't thank you all enough for voting this through on Greenlight and helping me get to this stage," Seabrook said in a Steam update. "Without you all I really wouldn't be writing this out today. Also, a massive thanks to Valve for their support."

Take a look at Prospekt in action in its trailer below:

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